1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly to an injection unit which enables a high-speed injection.
2. Background Art
FIG. 5 shows a conventional motor-driven injection molding machine. The injection molding machine 1 includes a front plate 3 and a rear plate 4 on a fixed base 2, which plates 3, 4 are coupled to each other by tie bars 5. A barrel 6 is fixed at the base portion to the front plate 3. A screw 7 is housed in the barrel 6. The screw 7 can be driven to rotate by means of a charging motor 8.
The base portion of the screw 7 is rotatably supported, via a bearing 9, by an intermediate plate 10. The intermediate plate 10 and the charging motor 8 can be reciprocated along the tie bars 5 in the direction of arrow A shown in FIG. 5 by means of a linear movement mechanism 13 which includes, for example, a ball screw 11 and a servo motor 12.
According to the injection molding machine 1 of FIG. 5, the intermediate plate 10, the charging motor 8, etc. are moved in the axial direction A together with the screw 7. Thus, the inertia is large as compared to the case of moving only the screw 7. The large inertia due to the concurrent movement of the intermediate plate 7, the motor 8, etc. has been a restriction on a high-speed injection. Further, the charging motor 8 is usually equipped with a not-shown encoder for detecting the number of revolutions of the motor 8. This entails the problem that a high acceleration upon a high-speed injection can damage the encoder.
Further, according to the injection drive mechanism shown in FIG. 5, the intermediate plate 10 moves while it is guided by the four tie bars 5. An error in the parallelism of the bars, between right and left bars or between upper and lower bars, upon assembling of the tie bars would therefore lead to an uneven sliding between the intermediate plate 10 and the tie bars 5, which would act as a resisting force against the injection driving force by the linear movement mechanism 13.
Furthermore, the conventional injection molding machine 1, with the provision of the tie bars 5 for supporting members that make a linear movement, e.g. the intermediate plate 10, can exert a high resistance to a tensile load in the back and forth direction (the A direction shown in FIG. 5). The conventional molding machine, however, has an insufficient rigidity to resist a load in the vertical direction or in the lateral direction (width direction of the machine) which may be caused by, for example, a misalignment from the injection shaft. This problem may be dealt with by making the tie bars 5 thicker. The use of thicker tie bars, however, necessitates an extra space for assembly, requires a larger-sized associated parts and incurs an increased cost.